Digester for use in paper making and allied industries



United States Patent 3,551,121 DllGESTER FOR USE IN PAPER MAKING AND ALLIED INDUSTRIES William T. De Long, West Manchester Township, York County, Pa., assignor, by mesne assignments, to Teledyne, Inc., Los Angeles, Calif., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 447,059, Apr. 9, 1965. This application Sept. 23, 1968, Ser. No. 761,870.

Int. Cl. B32b 15/00 11.8. Cl. 29-1961 2 Claims Broad, Preferred, percent percent 1 Maximum.

This application is in part a continuation of my copending application Ser. No. 447,059, filed Apr. 9, 1965, now abandoned. I

This invention relates to a digester for use in paper making and allied industries and to an overlay welding steel, i.e., a steel which can be weld deposited, which has improved corrosion resistance for paper mill digester service.

The manufacture of pulp for paper making involves the breaking down of wood chips to produce cellulose fibers in a digester under corrosive conditions. Digesters are normally made of mild steel due to cost factors. Because of corrosion the wall thickness gradually decreases and the digester must be either scrapped or overlaid with a protective material. Various alloy overlays have been used. The dilution of such weld deposited alloy overlays by the mild steel of the base plate must be regulated by controlling the deposit conditions in order that the deposit analysis will fall within the intended range. Weld deposits are usually of one layer and are often not very thick. Welding electrodes with less alloy may be used if two layers are deposited because of less dilution on the final layer, or conversely the rich electrodes will produce a more highly alloyed deposit if used in two layers. Weld deposits are usually made with horizontal beads on the vertical walls of the digester. Typically the dilution is in the range of to 40 percent, when manual stick electrodes are used, and 25 to 50 percent if a bare wire process such as submerged are or inert gasshielded metalarc welding is used.

One of the earliest corrosion resistant materials proposed for paper mill digester use was a wrought material with 15/20% chromium and 7.5/ 10% nickel, commonly known as 18-8. Weld deposits of this same analysis are attainable with normal dilution control with electrodes of the 309 type, the electrodes having about 25% chromium and 13% nickel and the deposit having about 16.7% chromium and 8.7% nickel with a 33% dilution.

However, weld deposits of the 18-8 composition have proven to be less satisfactory in service than weld deposits in the range of a 316 type. The popular and ac- 3,551,121 Patented Dec. 29, 1970 cepted material at present is a weld deposit with essentially an AISI Type 316 stainless steel composition, i.e., typically 17% Cr, 12% Ni and 2.2% M0. The carbon in the deposit is somewhat higher than normal for a Type 316, i.e., about 0.10% because of dilution with the mild steel base material, and the deposit is fully austenitic. An overlay of this analysis leaves much to be desired. The deposit is crack sensitive because it is fully austenitic Corrosion occurs at the cracks. The alloy is sensitive to variations in chemistry as shown by severe corrosion 1 problems in someareas of some heads.

In order to. get a ferrite bearing deposit of a chromium nickel molybdenumtype closely related toa 316 analysis, an electrode with 31% chromium, 16% nickel and 4.5% molybdenum has been proposed. It is capable of depositing a weld with 21.7% chromium, 11.2% nickel, 3.1% molybdenum and some ferrite in the case ,of a 30% dilution. In the case of a 45% dilution the typical deposit would be about 17% chromium, 8.8% nickel, 2.5% molybdenum, and contain less ferrite.

I have discovered a new deposit analysis which is greatly superior in corrosion resistance in paper mill digester service to the prior analyses.

A further important advantage of my invention is that it has good crack resistance by lvirtue of its ferrite content.

A further important advantage of my invention is that the addition of the expensive alloying element molybdenum is not required. No molybdenum is used in preferred compositions.

In the past there has been considerable speculation on What analyses Would prove most satisfactory for digester service, but the testing of these overlays is very time consuming and expensive. Completely satisfactory short term laboratory tests have not been developed; therefore testing is generally done by overlaying a small section in a digester which is in actual service. A test of a series of deposits in one digester gives comparative results. Such a test series was started several years ago. The more recent examination has revealed enough corrosion on the sample overlay steels so that now a clear evaluation can be made of their relative merits. The overlay deposit of my invention is outstanding, appreciably better than the early 188 type, the popular 316 type, or the ferrite bearing Cr-Ni-Mo type. The analysis of my new corrosion resistant alloy overlay weld deposit after removal from the digester is contrasted in the table below with the three prior art materials previously described.

Of these test patches exposed in the commercial digester for four years of continuous service, only the new alloy remained intact and it was rated as being in excellent condition.

The analytical limits of my new invention are accompanied by limits on the ferrite content of the deposit. These limits are based upon the well known fact that ferrite above about 4% is of great help in minimizing the cracking of austenitic stainless steel deposits. This benefit of ferrite is especially effective up to about 20% ferrite digester overlay surface. Even when present such hair line cracking is not necessarily harmful in paper mill digester service.

To obtain the benefits of my invention the analysis of the overlay deposit should be in the range:

With respect to molybdenum, excellent results have been secured without its presence, and while small amounts may confer small benefits in corrosion resistance, some tests indicate that amounts over two percent may actually reduce corrosion resistance. Molybdenum is therefore restricted to two percent maximum in our broad analysis, and further restricted to that which might be present as a residual, 0.6% maximum, in my preferred analysis.

With regard to other elements, the impurities carbon, phosphorus, sulfur and nitrogen should be kept low as in conventional chromium nickel stainless steels. The carbon in the material used for the overlay should be kept low. The carbon level of the overlay will depend primarily upon the carbon content of the base material and the amount of base material melted into the overlay, and with normal plate materials and overlay practices carbon should not exceed 0.15%, i.e., should be within the range of a Type 310 stainless steel. Silicon will normally be present in quantities up to 1.5% or more and manganese in quantities up to 2% or more in accordance with normal metallurgical deoxidizing practices.

The overlay or cladding analyses disclosed can be produced by the various methods of manual or automatic welding or metal deposition, for example, manual shielded arc, gas shielded arc, submerged arc, acetylene or plasma.

They can be produced, for example, with the aid of additive materials such as solid wire, tubes filled with powders;

metallic powders, plain fluxes and alloy bearing fluxes.

While I have described a present preferred embodiment of the invention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied within the scope of the following claims.

I claim:

1. A digester for use in paper making and allied industries comprising a clad structure having a steel base with a steel overlay completely covering the steel base, the steel overlay being resistant to fluids employed in the breaking down of wood chips to produce cellulose fibers and consisting essentially of 7 Percent Mo (max.) 2.0 Fe Balance and having a ferrite content of 4-60%.

2. A digester for use in paper making and allied industries as claimed in claim 1 comprising a clad structure having a steel base with a steel overlay completely covering the steel base, the steel overlay being resistant to fluids employed in the breaking down of wood chips to produce cellulose fibers and consisting essentially of:

HYLAND BIZOT, Primary Examiner 

